1. Field of the Invention
This invention relates generally to isolation amplifiers, and more particularly, to a transformer isolated amplifier employing multiple primary and secondary windings.
2. Description of the Prior Art
Isolation amplifiers are employed, for example, whenever it is necessary to measure low level signals in the presence of high common mode voltages so as to eliminate errors caused by disturbances on a common ground network, to avoid ground loops and inherent pick-up problems and to protect processing circuitry from damage by large common mode input and output voltage levels.
Transformer coupled isolation amplifiers commonly employed at least two transformers, one for coupling a carrier signal to an isolated input modulator and to provide power to an isolated input amplifier, and the other for coupling a modulated signal from the isolated input to a demodulator in the output section. Such an arrangement is shown and described in U.S. Pat. No. 3,754,193. However, using such an approach presents considerable difficulties in reducing the size, complexity, weight and cost of the amplifier along with a corresponding reduction in the reliability of the amplifier's operation.
To overcome some of these difficulties, isolation amplifiers have been designed which employ a single transformer to provide both an amplifier signal path and an energy path.
One such arrangement is shown and described in U.S. Pat. No. 3,988,690. An input amplifier stage is isolated from the main body of an amplifier by a single transformer having first and second primary windings and a secondary winding. The transformer provides both an energy path to the input stage and an amplifier signal path from the input stage using a modulated load technique. The secondary of the transformer is coupled across the input amplifier and includes a rectifier and filtering components which provide a floating D.C. voltage supply. The transformer rectifies the power on the secondary, and the power drain in modulated. Changes in current on the primary are monitored and used to detect the amplifier signal.
While this is a satisfactory arrangement with respect to modulating power, it does not represent an efficient and accurate way of separating power from the input signal since the zero offset point is load sensitive and varies with the "Q" of the transformer. Thus, this is not a satisfactory approach if D.C. coupling and accurate D. C. stability are important considerations.
An isolation amplifier arrangement shown and described in U.S. Pat. No. 4,066,974 represents a marked improvement with respect to D.C. stability. In this arrangement, an alternator coupled to a single secondary winding of a transformer alternately presents a voltage and a high impedance to the secondary winding. A first switching device couples the output of an input amplifier to a single primary winding when a high impedance is presented to the secondary winding. A second switching device couples the input of an output amplifier to the secondary winding when a high impedance is presented to the secondary winding.
Since an inductor cannot change current instantaneously, the voltage on the winding reverses in order to maintain its current each time a high impedance is presented to the secondary winding. Thus, the overall result of each voltage/high-impedance cycle is a positive power pulse having a trailing negative flyback pulse. It is this flyback pulse that is modulated with the input signal on the input side and then demodulated on the output side to yield the signal.
This arrangement suffers, however, from the disadvantage that power drawn the primary winding interferes with the amplitude of the negative flyback signal pulse; i.e. voltage drops in the primary winding alter the signal flyback portion.
This arrangement has additional disadvantages. Firstly, the arrangement employs a single flyback demodulator wherein a flyback rectifier places a negative voltage on a comparator. In order to get a voltage swing around zero volts, a resistor coupled to a positive voltage supply is used to buck out some of the negative voltage. This renders the output signal very sensitive to supply voltage variations; i.e. poor power supply rejection.
Secondly, this arrangement employs a complex scheme for producing the required positive and negative supply voltages using a single winding. The thus produced supply voltages are not very stable and result in interference with the actual signal.